1. Field of the Invention
This invention relates to a cooling structure for an electronic circuit component that is an improved cooling structure for a shunt resistor surface-mounted on a circuit board, and to an inverter apparatus using the same.
2. Description of the Related Art
In recent years, electric motors equipped with inverters have been increasingly popular because of the trend toward energy saving. These inverter apparatuses are provided with a current detecting circuit for detecting direct current to the power supply and electric current flowing through the electric motor, for the purpose of preventing the inverter breakage due to overcurrent or for the purpose of position-sensorless vector control of the electric motor.
A commonly-used structure of the just-described current detecting circuit is a current detecting circuit having a current sensor and a shunt resistor. However, because the current sensor has a large physical size, it leads to the problem of size increase of the inverter apparatus.
On the other hand, as for a current detecting device using the shunt resistor, a small-sized shunt resistor with an ultra-low resistance value of a few hundred micro-Ω that can be surface-mounted on a wiring board has been developed because of the technological advancement in recent years. Thus, it is advantageous in that a small-sized current detecting circuit can be constructed.
However, the shunt resistor generates heat with the square of the electric current value, so if the current-carrying capacity of the inverter apparatus becomes larger, the heat generation of the shunt resistor cannot be ignored, and the heat generation becomes a problem.
In particular, in the case of a surface mount-type flat panel-shaped shunt resistor, the generated heat is radiated mainly through the wiring board. Therefore, when the current-carrying capacity of the inverter apparatus becomes larger, the problem arises that the temperatures of the wiring board and the component parts in the surrounding region are increased by the influence of the heat generated by the shunt resistor.
In view of this, it is possible to conceive an inverter apparatus as shown in FIG. 7. For example, a heat sink 9 is provided on a solder side opposite to a component side of a circuit board 7 made of resin for cooling semiconductor switching elements 1 mounted on the component side of the circuit board 7. An insulating material 8 having a high thermal conductivity is interposed between the heat sink 9 and the circuit board 7, so as to release the heat generated from the shunt resistor 4, which is mounted on the component side of the circuit board 7, via a lead terminal 4b of the shunt resistor 4, a circuit pattern 6, and the circuit board 7 made of resin. Note that reference symbol 4a indicates a resistor of the shunt resistor 4.
In addition, for example, JP-A-2009-10082 (Patent Document 1) proposes, as a cooling method for the shunt resistor mounted on a wiring board other than the above-described method, a structure in which the heat generated by the shunt resistor is released from a radiator by providing the radiator on the opposite side to the side of the wiring board on which the shunt resistor is mounted.
[Patent Document 1] JP-A-2009-10082
As described above, the inverter apparatus having a great current-carrying capacity employs a structure in which the insulating material 8 having a high thermal conductivity is interposed between the circuit board 7 and the heat sink 9, and cooling is conducted by the radiator composed of the heat sink 9, as shown in FIG. 7, in order to release the heat from the heat-generating component parts such as power elements and shunt resistors. However, an extra cooling means for the shunt resistor 4 is required when the heat quantity generated by the shunt resistor 4 is great and cooling for the shunt resistor 4 is insufficient, even with the use of the cooling means for the shunt resistor 4 as described above.
Further cooling for the shunt resistor may be possible when the structure as in Patent Document 1 is used. However, a radiator for cooling the shunt resistor becomes necessary also on the upper side of the shunt resistor separately from the radiator for cooling the semiconductor switching element. This results in the problem of an increase in size of the inverter apparatus.